A biochar modified nickel-foam cathode with iron-foam catalyst in electro-Fenton for sulfamerazine degradation

A nickel-foam cathode modified by a self-nitrogen-doped biochar derived from waste giant reed was synthesized. The fabricated cathode (B@Ni-F) proved to be with high oxygen reaction reactive (ORR) reactivity and H₂O₂ selectivity (70.41%) owing to the enrichment of oxygen functional groups and pyridinic N when low-temperature pyrolyzed biochar was incorporated. The charge transfer resistance of B@Ni-F decreased to 7.18 Ω, which was 95.7 Ω for the original nickel-foam, proving by electrochemical impedance spectroscopy (EIS). Expectedly, Its H₂O₂ accumulation improved 14 times, thus making it comparable with commonly used electrodes like carbon cloth and graphite plate. Subsequently, B@Ni-F cathode and iron-foam (Fe-F) catalyst were firstly used in the electro-Fenton (EF) process for sulfamerazine (SMR) degradation. Double-functional polyphosphate electrolytes including tetrapolyphosphate (4-TPP), tripolyphosphate (3-TPP), pyrophosphate (PP) and Na₃PO₄ were compared with the conventional Na₂SO₄ electrolyte in EF for SMR degradation. The absolute rate constant for oxidation of SMR by ^[rad]OH was determined to be (3.4 ± 0.09) × 10⁹ M⁻¹ s⁻¹. SMR degradation enhancement in the presence of polyphosphate-based electrolytes is associated with bulk ^[rad]OH generation from Fe²⁺- polyphosphate ligand complexes via O₂ activation. The Fe²⁺-3-TPP complexes have relatively higher oxidation ability compared to Fe²⁺-PP, Fe²⁺-PO₄ species. A plausible SMR oxidation pathway is proposed based on the by-products detected by UPLC-MS/MS and density functional theory (DFT) calculations. The dominant SMR degradation pathway was hydroxylation of aniline residue of SMR, followed with the cleavage of –S–N– and then breakage of aromatic rings.